2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/185247We have derived new upper limits on several products (two at a time) of lepton number violating $λ'$-type couplings from the consideration of $ΔS = 2$ and $ΔB = 2$ box graphs. Each box contains one scalar lepton and one W-boson or one charged-Higgs-boson as internal lines. Most of these bounds are more stringent than previously obtained. Some of these product couplings drive $K_L$ (and some other $B_d$) decays to two charged leptons at enhanced rates. Some of them can explain the rare $K^+ \to π^+ ν\barν$ event recently observed at BNL.Latex, 6 pages, no figures; revised and published version with minor changes in the Table caption and a few cosmetic changes in the textHigh Energy Physics - Phenomenologytext